1. Field of the Invention
The invention relates to a workpiece stage of a resist curing device in which a workpiece on which a resist has been applied, such as a semiconductor wafer or the like, is held on a workpiece stage, and in which the resist is cured by heating and UV radiation. The invention furthermore relates to a workpiece stage of a resist curing device which is used in production processes for an integrated semiconductor circuit for resist treatment, in which a resist which has been applied to the wafer of silicon or the like is cured by UV radiation with a simultaneous increase of temperature by heating.
The workpiece stage in accordance with the invention is advantageously used especially as a new, high-efficiency workpiece stage which corresponds to an increase in the diameter of the wafer.
2. Description of the Prior Art
Conventionally, in production processes for an integrated semiconductor circuit the process for treatment of the resist which has been applied to the semiconductor wafer was a process in which the resist, after development with a simultaneous temperature increase, is irradiated by heating with UV radiation, in which the resist is cured, and in which thus its thermal resistance and plasma resistance and the like are increased (for example, Japanese patent HEI 4-78982, corresponding to U.S. Pat. No. 4,842,992). In the above described treatment, for each wafer, while UV radiation is performed, at the same time the temperature of the wafer treatment rack (of a workpiece stage) is controlled and the temperature increase and decrease described below is repeated.
In the above described process for treatment of a resist a series of treatments is carried out, specifically:                Treatment is started for example at 50° C. to 100° C.;        UV irradiation raises the temperature from 200° C. to 250° C.;        UV irradiation is stopped;        Cooling is performed again to a temperature from 50° C. to 100° C.; and        Treatment is concluded.In this case, the rate of temperature increase for a process-dictated reason must be 1° C./sec to 2° C./sec. It is desirable for the rate of temperature decrease to be higher in order to increase the throughput; but, in practice, the decrease is roughly 3° C./sec.        
In order to reliably transfer the above described rapid temperature increase and decrease to the wafer, advantageously a process is undertaken in which the wafer is held by vacuum suction on a metallic workpiece stage with good thermal conductivity. Specifically, for this workpiece stage, one is used with a temperature control means which has a heating means and a cooling means. It is heated by a heating apparatus and cooled with cooling water. In this way, the control of the above described temperature increase and decrease is carried out.
As generic art, FIG. 1 shows a normal workpiece stage in an overall perspective view. The workpiece stage WS has a circular shape like the wafer. On the surface of the workpiece stage WS, there are several annular vacuum suction grooves A. The wafer to which the vacuum is supplied and which was put in place is held by the surface of the workpiece stage WS by vacuum suction. Cartridge heaters B are inserted into the workpiece stage. The surface of the workpiece stage is provided with a non-electrolytic nickel coating 2.
In FIG. 6(a), the workpiece stage WS is shown in a side view, FIG. 6(b) shows a cartridge heater B in a cross section. The side of the workpiece stage WS is provided with several through openings 3 for insertion of heaters, into which rod-shaped cartridge heaters B are inserted. Cartridge heaters B have a cartridge 7 which corresponds to the length of the through opening, and a heating wire 8. When current is supplied, the cartridge heater B is heated, and the workpiece stage WS is heated by thermal conduction.
Within the workpiece stage, between the through openings 3 for inserting the heaters B, there are through openings 4 for cooling water through which cooling water flows, and a vacuum supply line (not shown in the drawings) by which a vacuum is drawn through the vacuum suction grooves 5. The passages through which cooling water flows are connected to one another on the bottom of the workpiece stage WS by bridge pipelines 6. The cooling water is introduced via the insertion openings E at two points and drained by drain openings F at two points.
However, recently there has been a tendency to increase the diameter of the wafer from 200 mm to 300 mm, by which the diameter of the workpiece stage must be increased accordingly. When the diameter of the workpiece stage increases, the following disadvantages arise:
Conventionally, the material of the above described workpiece stage of the resist curing device has been low-oxygen copper with good thermal conductivity. By increasing the diameter of the carrier in order to handle a wafer with a large diameter of 300 mm in a suitable manner, due to repetition of the process of increasing and decreasing the temperature of the carrier, a warp was formed due to bowing of the carrier (FIG. 2), resulting in the disadvantage that the wafer could no longer be held by drawing a vacuum.
For example, when the temperature is increased and decreased roughly 100 times between 50° C. and 250° C. (2.1° C./sec rate of temperature increased and 3° C./sec rate of temperature decrease) in a workpiece stage WS of low-oxygen copper with a diameter of 320 mm and a thickness of 17 mm, as is shown in FIG. 2, the disadvantage arises that, with respect to the center area of the workpiece stage, an warp of 1.2 mm forms in the peripheral area.
The inventors have ascertained by tests that a wafer cannot be reliably held by drawing a vacuum if the warp of the peripheral area with respect to the center area of the workpiece stage greater than 0.5 mm. If the wafer cannot be held on the workpiece stage by vacuum, the temperature of the carrier cannot be conducted exactly to the wafer. It is possible for a disadvantage to arise in the curing treatment of the resist.